Sheet registration systems deliver sheets of all kinds to specified positions and angles for subsequent functions within printers, copiers and other printing machines. The subsequent functions may include transferring an image to the sheet, stacking the sheet, slitting the sheet, etc. Conventional registration systems correct for skew, lateral offset, and process errors. “Skew” is the angle the leading edge of a sheet being transferred differs from perpendicular to the desired direction of transfer. “Lateral offset” or “cross process offset: is the lateral misalignment of the sheet being transferred with respect to the desired transfer path. “Process” relates to the timing of the sheet within the printing machine such that the sheet arrives at various destinations at the proper times.
Examples of skew contributors include (i) the angle at which a sheet is supplied into the sheet drive apparatus, (ii) skew induced when the sheet is acquired by the feeder, and (iii) drive roller velocity differences between drive rollers on opposite ends of a common drive shaft. Typical reasons for lateral offset include improper sheet supply location and sheet drive direction error. Sheet drive direction error is caused by the sheet drive shafts not being perpendicular to the intended sheet drive direction. This is a result of tolerances and excess clearance between drive shafts and frames, sheet transport mounting features and machine frames and machine module to module mounting. A typical reason for a process error may be an incorrect nip drive speed.
In present day high speed copiers and printers, active registration systems are used to register the sheets accurately. In an active registration system, a sheet is passed over sensor arrays from which the sheet skew, lateral offset, and process errors are calculated. Skew is corrected in some registration systems by rotating drive rollers on opposite ends of a common drive axis at different velocities. Lateral offset may be corrected, for example, by moving the rollers in unison to one side or another. Process errors may be corrected, for example, by driving the rollers faster or slower.
Upon completion of the registration process corrects for skew, lateral offset, and process errors the sheet is correctly aligned along a desired transfer path and ready to receive an image within a pre-defined image area. In a typical application, the predefined image area is the area defined within 1 inch margins or borders of the sheet. Following the registration process each sheet is delivered to an imaging station where an image is created on the surface of the sheet. In certain printing machines, the sheet is then passed through a fuser that fuses the image to the sheet. It is typically desirable for the image to be centered within the predefined image area.
Duplex printing generally refers to the process of printing an image on a first side and a reverse side of a single sheet. The duplex printing process typically begins with a sheet being fed through a sheet feeder and into a transfer path. The sheet then encounters a sheet registration system that collects information concerning the orientation of the sheet, such as skew and lateral offset, and may re-orient the sheet to place it in better position for imaging. Thereafter, the sheet is moved to an imager located downstream from the registration system. The imager transfers developed images from a photoreceptor to the sheet, thus creating an image on the sheet. After this, the sheet is passed on to a fusing station where the image is fused to the first side of the sheet. During this first imaging process, the first side of the sheet is the upper side and the reverse side of the sheet is the lower side.
After an image is created on the first side of the sheet, the duplex printing process continues as the sheet is inverted in a sheet inverter such that the first side becomes the lower side and the reverse side becomes the upper side. The sheet is then moved along a duplex path to an inverter. The inverter flips the sheet such that what was the leading edge of the sheet during the first imaging process becomes the trailing edge of the sheet during the second imaging process. After inversion, the sheet is returned to the transfer path for re-registered of the sheet for the second imaging process. After the sheet is re-registered, the sheet is passed through the imaging and fusing process, thereby placing an image within a second predefined area on the reverse side of the sheet. In the end, it is desirable for the first pre-defined image area to match the second pre-defined image area such that the image on the first side appears within the same sheet boundaries as the image on the reverse side when the sheet is inspected by holding the sheet up to a light. The intended alignment of the image on the first side with the image on the second side is often referred to as see-through registration.
Improper sheet size is a major factor contributing to misalignment of images on opposite sides of a sheet of paper during the duplex printing process. An improper sheet size is often the result of a sheet of paper that is (i) non-rectangular or (ii) wider or narrower than intended (e.g., slightly greater than or slightly less than 8½″ wide). Improper sheet size is generally attributable to paper manufacturing defects, large manufacturing tolerances in paper size, or changes in size of the paper during fusing before the second imaging process.
An example of the problem created by an improper sheet size is shown with reference to FIG. 5. In FIG. 5, the bold outer perimeter 52 represents a non-rectangular sheet of paper 52. The non-rectangular character of the sheets is exaggerated in FIG. 5 over that of a typical non-rectangular sheet for emphasis. As shown in FIG. 5, the leading edge 54 of the sheet is not parallel with the trailing edge 56 of the sheet. A first image is printed on the first side of the sheet within a first predefined image area 60 which is defined in FIG. 5 by the solid line on the sheet. The first image area 60 includes a border 61 that is aligned with the leading edge of the sheet. After the first image is created during the duplex printing process, the sheet is flipped such that the former leading edge 54 becomes the trailing edge and vice-versa. Then, a second image is printed on the reverse side of the sheet within a second predefined image area 62 which is defined in FIG. 5 by the dotted line on the sheet. The second image area 62 includes a border 63 that is aligned with the new leading edge of the sheet 56 (which was formerly the trailing edge). As shown in FIG. 5, the first image area 60 on the first side of the sheet is not aligned with the second image area 62 on the reverse side of the sheet, creating duplex image misalignment when a see-through inspection of the image is made.
Although FIG. 5 represents a situation where the sheet is non-rectangular, similar see-through registration problems occur during the duplex printing process when the distance from the leading edge to the trailing edge of the sheet is longer or shorter than expected. Accordingly, it would be desirable to provide a printing system capable of accurately producing duplex images where a first side image is aligned with a reverse side image when a see-through inspection of the sheet is made.